Note [Do not eta-expand join points]

Similarly to CPR (see Note [Don't w/w join points for CPR] in
GHC.Core.Opt.WorkWrap), a join point stands well to gain from its outer binding's
eta-expansion, and eta-expanding a join point is fraught with issues like how to
deal with a cast:

    let join $j1 :: IO ()
             $j1 = ...
             $j2 :: Int -> IO ()
             $j2 n = if n > 0 then $j1
                              else ...

    =>

    let join $j1 :: IO ()
             $j1 = (\eta -> ...)
                     `cast` N:IO :: State# RealWorld -> (# State# RealWorld, ())
                                 ~  IO ()
             $j2 :: Int -> IO ()
             $j2 n = (\eta -> if n > 0 then $j1
                                       else ...)
                     `cast` N:IO :: State# RealWorld -> (# State# RealWorld, ())
                                 ~  IO ()

The cast here can't be pushed inside the lambda (since it's not casting to a
function type), so the lambda has to stay, but it can't because it contains a
reference to a join point. In fact, $j2 can't be eta-expanded at all. Rather
than try and detect this situation (and whatever other situations crop up!), we
don't bother; again, any surrounding eta-expansion will improve these join
points anyway, since an outer cast can *always* be pushed inside. By the time
CorePrep comes around, the code is very likely to look more like this:

    let join $j1 :: State# RealWorld -> (# State# RealWorld, ())
             $j1 = (...) eta
             $j2 :: Int -> State# RealWorld -> (# State# RealWorld, ())
             $j2 = if n > 0 then $j1
                            else (...) eta